Abstract: Process for preparing polyoxymethylene dimethyl ethers having ?3 oxymethylene units (OMEn?3), comprising the steps: (i) introduction of formaldehyde, methanol and water into a reactor R and reaction to give a reaction mixture containing formaldehyde, water, methylene glycol, polyoxymethylene glycols, methanol, hemiformals, methylal (OMEn=1) and polyoxymethylene dimethyl ethers (OMEn>1); (ii) introduction of the reaction mixture into a reactive distillation column K1 and separation into a low boiler fraction F1 containing formaldehyde, water, methylene glycol, polyoxymethylene glycols, methanol, hemiformals, methylal (OMEn=1) and polyoxymethylene dimethyl ethers having from 2 to 3 oxymethylene units (OMEn=2-3) and a high boiler fraction F2 containing polyoxymethylene dimethyl ethers having more than two oxymethylene units (OMEn?3).